Abstract
This thesis covers the experimental investigation of four-level pulse amplitude modulation (PAM4) and discrete multi-tone transmission (DMT) and their different flavours as cost and power efficient solutions for next generation of intra- and inter-data center interconnects (DCIs). In addition, it studies different transmitter and receiver equalization concepts for PAM4.
On the road towards a lane rate of 100Gb/s for intra-DCIs, this thesis investigates the transmission performance of both modulation formats in combination with a vertical cavity surface emitting laser (VCSEL) at 1530nm and with an electro-absorption modulated laser (EML) at 1300nm. The investigations resulted in successful demonstrations of 84Gb/s PAM4 and 74.7Gb/s DMT over up to 1.6km and 5km standard single mode fiber (SSMF), respectively, with the VCSEL and 112Gb/s over 10km with the EML.
For the scenario of inter-DCIs with a transmission distance of up to 80km SSMF, this thesis evaluates and compares PAM4 and DMT at lane rates of up to 56Gb/s. In addition, part of the PAM4 evaluation is done in real-time with prototype PAM4 PHYs, demonstrating the commercial feasibility of the transmission system. For modulation, a lithium niobate Mach-Zehnder modulator (MZM) is used to benchmark the transmission system as well as a silicon photonic ring-modulator in combination with PAM4 modulation investigated as a potential low-cost modulator. This thesis studies also different distortions of the transmission system such as optical noise, chromatic dispersion, fiber nonlinearities, channel crosstalk and optical bandwidth limitations and its influence on the different modulation formats. Finally, the data rate for PAM4 is enhanced towards 112Gb/s for a potential follow-up generation and its performance dependent on different equalizer approaches presented.

DTU Fotonik's research covers a broad spectrum of fields within Photonics Engineering.
It ranges from basic scientific explorations into light-matter interaction, via communication technologies,
lasers and sensors, to collaborations with architects and designers on LED light sources of the future.The department focuses on the application oriented uses of our research and on solving the societal challenges that we face.